It is known in the art to prolong the life of spark plug electrodes by attaching precious or noble metal tips to their firing ends. Some of the earliest examples of this technology are seen in U.S. Pat. No. 2,296,033 issued Sep. 15, 1942 to Heller, and in British Patent Specification No. 479,540 published in 1938 to Powell et al. The Heller patent teaches the attachment of precious metal tips to ground and center electrodes formed of much less expensive materials. The precious metal tips are comprised of corrosion resistant materials, including platinum alloys such as platinum-rhodium, platinum-iridium and platinum-ruthenium. Similarly, the Powell reference discloses the use of platinum, iridium, ruthenium, osmium and alloys thereof, including iridium-rhodium, for use as firing tips for spark plug electrodes. In the time since this and other early designs, there have sprung numerous other inventions attempting to utilize the corrosion and erosion resistant properties of noble and other precious metals.
For many years, platinum was the precious metal of choice for spark plug electrode firing tips, as evidenced by the numerous patents describing its use. During recent years, however, numerous other noble metals and noble metal alloys have become more frequently utilized; one of which is iridium. Iridium can be relatively inexpensive, when compared to other noble metals, and has the rather high melting point of approximately 2410° C. Though many benefits exist regarding the use of iridium, it is sometimes a challenge to work the noble metal, as it has a tendency to crack under mechanical pressure and deformation. In order to overcome this and other challenges, various iridium-alloys have been developed with the hope of imparting certain, desirable characteristics to the metal. An example of such an alloy is taught in U.S. Pat. No. 6,094,000 issued Jul. 25, 2000 to Osamura et al. In this reference there is disclosed an Ir—Rh alloy whose relative percentages of iridium and rhodium vary according to one of several embodiments.
Attachment of iridium and other such firing tips is commonly done by welding and, in particular, laser welding of the tip to a center electrode. Typically, the tip is in the form of a segment of cylindrical wire. However, other tip configurations also exist for use with other attachment techniques. See, for example, U.S. Pat. No. 6,614,145 to Fleetwood et al. in which an iridium tip with an enlarged head is attached by swaging and brazing the tip within a blind hole of an upper electrode.